This application claims benefit of priority to Japanese Application No. JP 2002-67506 filed Mar. 12, 2002, the entire content of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to motor control device that performs flux-weakening control of a motor, and more particularly relates to a motor control device applied to a permanent magnet reluctance motor that generates a combined torque of torque produced by a permanent magnet and reluctance torque.
2. Description of the Related Art
Conventionally, when performing inverter drive of a permanent magnet motor or reluctance motor used in applications such as electric trains or electrical automobiles, flux-weakening control is often performed so as to make the motor terminal voltage below the maximum voltage capable of being output by the inverter, in the case of fixed output operation during high-speed rotation.
In flux-weakening control of a permanent magnet motor, typically, an armature current i.e. a so-called minus d axis current is made to flow such that the magnetic flux produced by the current flowing in the armature is in the opposite direction to the magnetic flux of the permanent magnet.
Usually, also, flux-weakening control of a reluctance motor which has an axis (q axis) of larger inductance and an axis (d axis) of smaller inductance is performed by reducing the q axis current, which has the largest flux-weakening effect.
However, in the case of performing flux-weakening control of a permanent magnet reluctance motor in which a combined torque of the torque produced by permanent magnet flux and reluctance torque is generated, when the current for flux-weakening control is made to flow fixed on either axis of the d axis or q axis, depending on the magnitude of the current amplitude that is to be made variable by the torque that is desired to be output, a flux-weakening effect may not be obtained, with the result that control instability is produced in which the motor terminal voltage cannot be controlled to below the inverter output maximum voltage.
It is possible that the same phenomenon may occur also in so-called embedded permanent magnet motors, in which a combined torque of reluctance torque and permanent magnet torque dependent on the shape of a permanent magnet embedded in the interior of the rotor core is output.
As a method of solving this problem, the present inventors have previously proposed (Laid-open application number 2001-197800 in Japan, U.S. application Ser. No.: 09/751,387) a device for controlling a permanent magnet reluctance motor whereby stable and effective flux-weakening control is made possible irrespective of the torque output condition by making it possible to vary the direction of current flow for flux-weakening control in accordance with the magnitude of the torque instruction or current instruction in a permanent magnet reluctance motor.
However, this system is a method of solution that is principally aimed at stability and effectiveness of flux-weakening control and does not necessarily take into consideration ease of tracking a torque to the torque instruction value; thus it suffered from the problems of system control tending to become unstable due to runaway caused by a larger than anticipated torque being produced if the deviation between the actually generated torque and the torque instruction value became large, or to due to generation of mechanical vibrations.
Accordingly, one object of the present invention, when applied to a permanent magnet reluctance motor that generates a combined torque of torque produced by permanent magnet flux and reluctance torque, is to provide a novel motor control device whereby stable and effective flux-weakening control can be performed while outputting a torque that closely tracks the torque instruction value, irrespective of the torque output condition.
In order to achieve the above object, the present invention is constituted as follows. Specifically, in a motor control device comprising flux-weakening angle setting means that, when flux-weakening control is performed wherein the current instruction value is corrected such that the terminal voltage of the motor does not exceed the maximum voltage of the inverter output, variably sets an angle of flux-weakening whereby correction is effected by the current instruction value in accordance with a torque instruction, the flux-weakening angle setting means sets the flux-weakening angle in a direction in which the torque is practically invariant.
With the present invention, when applied to a permanent magnet reluctance motor that generates a combined torque of torque produced by permanent magnet flux and reluctance torque, it is made possible to variably set the direction of the current flowing for flux-weakening control taking into account not only the torque instruction but also the magnitude of the flux-weakening current; in this way, stable and effective flux-weakening control can be performed while outputting a torque that closely tracks the torque instruction value, irrespective of the torque output condition.
Also in order to achieve the above object, the present invention is further constructed as follows. Specifically, a motor control device comprising flux-weakening angle setting means that, when flux-weakening control is performed wherein the current instruction value is corrected such that the terminal voltage of the motor does not exceed the maximum voltage of the inverter output variably sets an angle of flux-weakening whereby correction is effected by said current instruction value in accordance with a torque instruction value, comprises:
single pulse waveform control means that controls the current such that a desired torque is obtained by altering the voltage phase, by outputting a single-pulse waveform that turns an inverter switching element ON/OFF once only in every cycle of the output frequency; and
means for altering the voltage phase that sets the angle of flux-weakening in accordance with the torque instruction in a direction wherein the torque is practically invariant and such that the current feedback component in the direction perpendicular to the angle of flux-weakening becomes zero.
With the present invention, when applied to a permanent magnet reluctance motor that generates a combined torque of torque produced by permanent magnet flux and reluctance torque, it is made possible to variably set the direction of the current flowing for flux-weakening control taking into account not only the torque instruction but also the magnitude of the flux-weakening current; in this way, stable and effective flux-weakening control can be performed while outputting a torque that closely tracks the torque instruction value, irrespective of the torque output condition.
Also, thanks to the adoption of a single pulse waveform, the inverter voltage utilization rate can be improved, device costs can be lowered and higher efficiency can be achieved.